1. Field of the Invention
The invention relates generally to the field of post tension concrete reinforcing devices and systems. More particularly, the invention relates to structures for anchors used in such concrete reinforcing systems.
2. Background Art
Structural concrete is capable of carrying substantial compressive load, however, concrete is unable to carry significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces on a concrete structure.
The basic principle of concrete reinforcement is simple. In pre-stressing, which is one of two basic types of reinforcement, reinforcing rods of high tensile strength wires are stretched a certain amount and then high-strength concrete is placed around the reinforcing rods. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. The other type of reinforcement, called post-tensioning, follows the same general principle, but the reinforcing rods (called “tendons”) are held loosely in place while the concrete is placed around them. The tendons are then stretched by hydraulic jacks and are securely anchored into place. Prestressing is typically performed within individual concrete members at the place of manufacture. Post-tensioning is generally performed as part of the structure on the construction site.
A typical tendon tensioning anchor system for post-tensioning operations, includes a pair of anchors for anchoring the two ends of the tendons suspended therebetween. In the course of installing the tendon and anchors in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of the tendon for applying a predetermined amount of tension to the tendon. When the desired amount of tension is applied to the tendon, wedges, threaded nuts, or the like, are used to capture the tendon and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition, thus applying tensile force on the tension to the anchors.
Metallic components, such as tendons, disposed within concrete structures may be come exposed to many corrosive elements, such as de-icing chemicals, sea water, brackish water, or spray from these sources, as well as salt water. If such exposure occurs, and the exposed portions of the anchor and tendon suffer corrosion, then the anchor may become weakened due to this corrosion. The deterioration of the anchor and tendon can cause the tendons to slip, thereby losing the compressive effects on the structure, or the anchor can fracture. In addition, the large volume of by-products from the corrosive reaction is often sufficient to fracture the surrounding structure. These elements and problems can be sufficient so as to cause a premature failure of the post-tensioning system and a deterioration of the structure.
A typical post-tension assembly, therefore, includes a liquid tight covering or sheathing on its exterior surface. Some anchors are encapsulated in a moisture proof material such as plastic. An example of such an encapsulated post tension reinforcing system is described in U.S. Pat. No. 5,072,558 issued to Sorkin et al. The system disclosed in the '558 patent includes a tendon having an exposed end protruding from a sheath. The exposed end of the tendon is typically fitted through an extension tube. The extension tube has a diameter slightly larger than sheath, such that one end of the extension tube may overlie the sheath. The opposite end of the extension tube fits over, and communicates with, a rear tubular portion of an anchor. The rear tubular member includes an aperture which communicates with a frontal aperture. The frontal aperture defines a cavity or bore in which anchoring wedges are received.
As known in the art, the tendon is disposed through the extension tube and through the anchor wedge receiving bore. The end of the extension tube is sealed to the outer surface of the sheath. After the tendon extends through the frontal aperture, and assuming the far end of the tendon is fixed in place, tension is applied to the tendon, typically by use of a hydraulic jack. While applying this tension, wedges are forced in place on both sides of tendon within the wedge receiving bore. Once in place, teeth on the wedges operate to lock the tendon in a fixed position with respect to the anchor. Thereafter, the tension supplied by the hydraulic device is released and the excess tendon extending outward from the anchor is cut by a torch or other known device. The wedges thereafter prevent the tendon from releasing its tension and retracting inward with respect to the anchor. Moreover, the tension remaining on the tendon provides additional tensile strength across the concrete structure.
It has been determined that the wedge receiving cavity in the anchor body known in the art crated many problems. The wedge receiving bore in the anchor body is typically of a constantly diminishing diameter extending from a forward end of the anchor body to a rearward end of the anchor body. This constantly diminishing diameter is formed during the casting of the anchor body. However, the narrow diameter end of the wedge receiving bore creates problems with the installation of sheathed tendons. When the anchor body is used in the formation of intermediate anchorages, for example, it is often necessary to move the anchor body over a very long length of sheathed tendon. If there is insufficient clearance between the narrow diameter end of the cavity and the outer diameter of the sheathed portion of the tendon, nicks, abrasions, and cuts can occur in the corrosion-resistant sheathing. As such, the integrity of the anchorage system is impaired. Furthermore, there are circumstances where the sheathing diameter may exceed expected tolerances and will prevent the anchor body from easily sliding along the length of the tendon so as to assume its position as an intermediate anchorage. Additionally, in recent years, there has been a tendency to increase the thickness of the sheathing so as to facilitate greater protection of the tendon from corrosive elements. It should be noted that similar problems can occur at a “live end” terminal anchor, the live end being the end of the tendon that is pulled or stretched to apply tension to the tendon.
An easy solution to the foregoing problems would be to expand the diameter of the wedge receiving bore so as to avoid the aforementioned problems. However, if the overall diameter of the bore is expanded, then conventional (standard size and taper) wedges cannot be used. Other problems may occur if larger or non-standard size wedges or if irregular wedges are used. If the wedge receiving bore were enlarged, then the wedge components would have to be replaced in all such post-tension anchor systems.
It is also known in the art to drill out or ream the narrow diameter end of the wedge receiving bore so as to produce a portion of generally constant diameter. However, drilling and reaming have some limitations. First, drilling or reaming can be very expensive in comparison with the casting of the anchors. Furthermore, drilling or reaming of a constant diameter portion in the anchor body can create burrs and deformations which could potentially cut the sheathing of the tendon and cause adverse corrosion-protection results. Finally, drilling or reaming the narrow portion of the wedge receiving bore can intrude into the wedge-contact area so as to cause uneven and irregular contact between the wedges and the wall of the cavity. Such irregular contact may weaken the anchoring system.
One solution to the foregoing is described in U.S. Pat. No. 6,017,165 issued to Sorkin. An anchor body disclosed in the '165 patent includes an internal wedge-receiving cavity. The cavity has a first portion of constantly diminishing diameter extending inwardly from one end of the anchor body. The first portion has an angle of taper with respect to a center line of the cavity. The cavity has a second portion extending inwardly from an opposite end of the anchor body. The first portion and the second portion are coaxial and communicate with each other. The second portion has an angle of taper which is less than the first portion. The first and second portions are cast with the anchor body. Other patents issued to Sorkin disclose variations of the same general concept, namely that the wedge receiving cavity is divided into a first portion and a second portion, wherein the second portion has a different taper angle than the first portion, such that a minimum internal diameter of the wedge receiving bore is at least large enough to enable free passage of a sheathed tendon therethrough.
One limitation to the anchors disclosed in the various Sorkin patents is the cost of casting the anchor to have more than one taper angle in the wedge receiving bore. It has also been determined that prior art wedges may be more massive, and have more uneven distribution of axial stresses to the anchor base or plate than may be considered optimal. Accordingly, there is a need for an anchor for post tension concrete reinforcing systems which more evenly distributes stress to the anchor base, and which is less expensive to manufacture.